Surveillance system having acoustic magnetomechanical marker

ABSTRACT

An article surveillance system having an acoustic magnetomechanical marker is adapted, when armed, to resonate at a frequency provided by an incident magnetic field applied within an interrogation zone. The marker is a strip of magnetostrictive ferromagnetic material disposed adjacent to a hard magnetic element which, upon being magnetized, magnetically biases the strip and arms it to mechanically vibrate at said frequency. A substantial change in acoustic output level of the marker at the resonant frequency provides the marker with signal identity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to article surveillance systems and markers foruse therein. More particularly, the invention provides a ferromagneticmetal marker that enhances the sensitivity and reliability of thearticle surveillance system.

2. Description of the Prior Art

The problem of protection of articles of merchandise and the like,against theft from retail stores has been the subject of numeroustechnical solutions. Among these, a tag or marker is secured to thearticle to be protected. The marker responds to an interrogation signalfrom transmitting apparatus situated either at the exit door of thepremises to be protected, or at the aisleway adjacent to the cashier orcheck out station. A receiving apparatus on the opposite side of theexit or aisleway from the transmitting apparatus, receives a signalproduced by the marker in response to the interrogation signal. Thepresence of the response signal indicates that the marker has not beenremoved or deactivated by the cashier, and that the article bearing itmay not have been paid for or properly checked out.

Several different types of markers have been disclosed in theliterature, and are in use. In one type, the functional portion of themarker consists of either an antenna and diode or an antenna andcapacitors forming a resonant circuit. When placed in an electromagneticfield transmitted by the interrogation apparatus, the antenna-diodemarker generates harmonics of the interrogation frequency in thereceiving antenna; the resonant circuit marker causes an increase inabsorption of the transmitted signal so as to change the signal in thereceiving coil. The detection of the harmonic or signal level changeindicates the presence of the marker. With this type of system, themarker must be removed from the merchandise by the cashier. Failure todo so indicates that the merchandise has not been properly accounted forby the cashier.

A second type of marker consists of a first elongated element of highmagnetic permeability ferromagnetic material disposed adjacent to atleast a second element of ferromagnetic material having highercoercivity than the first element. When subjected to an interrogationfrequency of electromagnetic radiation, the marker causes harmonics ofthe interrogation frequency to be developed in the receiving coil. Thedetection of such harmonics indicates the presence of the marker. Onemethod of deactivation of the marker is accomplished by changing thestate of magnetization of the second element. Thus, when the marker isexposed to a dc magnetic field, the state of magnetization in the secondelement changes and, depending upon the design of the marker being used,either the amplitude of the harmonics chosen for detection issignificantly reduced, or the amplitude of the even numbered harmonicsis significantly changed. Either of these changes can be readilydetected in the receiving coil.

Ferromagnetic harmonic generating markers are smaller, contain fewercomponents and materials, and are easier to fabricate thanresonant-circuit or antenna-diode markers. As a consequence, theferromagnetic marker can be treated as a disposable item affixed to thearticle to be protected and disposed of by the customer. Such markersmay be readily deactivated by the application of a dc magnetic fieldpulse triggered by the cashier. Hence, handling costs associated withthe physical removal requirements of resonant-circuit and antenna-diodemarkers are avoided.

One of the problems with harmonic generating, ferromagnetic markers isthe difficulty of detecting the marker signal at large distances. Theamplitude of the harmonics developed in the receiving antenna is muchsmaller than the amplitude of the interrogation signal, with the resultthat the range of detection of such markers has heretofore been limitedto aisle widths less than about three feet. Another problem withharmonic generating, ferromagnetic markers is the difficulty ofdistinguishing the marker signal from pseudo signals generated by beltbuckles, pens, hair clips and other ferromagnetic objects carried byshoppers. The merchant's fear of embarrassment and adverse legalconsequences associated with false alarms triggered by such pseudosignals will be readily appreciated. Yet another problem with suchferromagnetic markers is their tendency to be deactivated or reactivatedby conditions other than those imposed by components of the system.Thus, ferromagnetic markers can be deactivated purposely uponjuxtaposition of a permanent magnetic or reactivated inadvertently bymagnetization loss in the second ferromagnetic element thereof. Forthese reasons, article surveillance systems have resulted in higheroperating costs and lower detection sensitivity and operatingreliability than are considered to be desirable.

SUMMARY OF THE INVENTION

The present invention provides a marker capable of producing identifyingsignal characteristics in the presence of a magnetic field appliedthereto by components of an article surveillance system. The marker hashigh signal amplitude and a controllable signal signature and is notreadily deactivated or reactivated by conditions other than thoseimposed by components of the system.

In addition, the invention provides an article surveillance systemresponsive to the presence within an interrogation zone of an article towhich the marker is secured. The system provides for high selectivityand is characterized by a high signal-to-noise ratio. Briefly stated,the system has means for defining an interrogation zone. Means areprovided for generating a magnetic field of predetermined frequencywithin the interrogation zone. A marker is secured to an articleappointed for passage through the interrogation zone. The markercomprises a strip of magnetostrictive ferromagnetic material adapted tobe magnetically biased and thereby armed to resonate mechanically at afrequency within the frequency band of the incident magnetic field. Amagnetic element with low coersive field, disposed adjacent to the stripof magnetostrictive material, is adapted, upon being magnetized, to armthe strip to vibrate at its mechanical resonant frequency. The strip ofmagnetostrictive material has a magnetomechanical coupling factor, k,greater than 0, where k=√(1-f_(r) ² /f_(a) ²), f_(r) and f_(a) being theresonant and antiresonant frequencies, respectively. Upon exposure tosaid magnetic bias field the marker is characterized by a substantialchange in mechanical vibration when the applied ac field is at themarkers resonant frequency thus providing the marker with signalidentity. A detecting means detects the change in mechanical vibrationof the magnetostrictive ferromagnetic material at its resonant frequencyby monitoring the amplitude changes of acoustic signals produced by suchmechanical vibration, and distinguishing such amplitude changes fromthose at frequencies other than the resonant frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiment of the invention and theaccompanying drawings in which:

FIG. 1 is a block diagram of an article surveillance systemincorporating the present invention;

FIG. 2 is a diagrammatic illustration of a typical store installation ofthe system of FIG. 1;

FIG. 3 is a graph showing the voltage induced by mechanical energyexchange of a strip of amorphous magnetostrictive ferromagnetic materialover a preselected frequency range;

FIG. 4 is an isometric view showing components of a marker adapted foruse in the system of FIG. 1;

FIG. 5 is an isometric view of an alternate construction of a markeradapted for use in the system of FIG. 1;

FIG. 6 is a schematic electrical diagram of an interrogation anddetection scheme comprising part of the article surveillance system ofFIG. 1; and

FIG. 7 is a schematic electrical diagram of an interrogation anddetection scheme comprising a part of an alternative embodiment of thearticle surveillance system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The magnetomechanical marker of article surveillance system 10 can befabricated in a number of diverse sizes and configurations. As aconsequence, the invention will be found to function with many varietiesof surveillance systems. For illustrative purposes the invention isdescribed in connection with an antipilferage system wherein articles ofmerchandise bearing the markers are surveyed by the system to preventtheft of the merchandise from a retail store. It will be readilyappreciated that the invention can be employed for similar and yetdiversified uses, such as the identification of articles or personnel,wherein the marker and the system exchange magnetomechanical energy sothat the marker functions as (1) personnel badge for control of accessto limited areas, (2) a vehicle toll or access plate for actuation ofautomatic sentrys associated with bridge crossings, parking facilities,industrial sites or recreational sites, (3) an identifier for checkpoint control of classified documents, warehouse packages, library booksand the like, (4) product verification. Accordingly, the invention isintended to encompass modifications of the preferred embodiment whereinthe resonant frequency of the marker provides animate or inaminateobjects bearing it with signal identity.

Referring to FIGS. 1, 2 and 4 of the drawings, there is shown an articlesurveillance system 10 responsive to the presence of an article withinan interrogation zone. The system 10 has means for defining aninterrogation zone 12. A field generating means 14 is provided forgenerating a magnetic field of desired frequency within interrogationzone 12. A marker 16 is secured to an article 19 appointed for passagethrough the interrogation zone 12. The marker 16 is comprised of a strip18 of amorphous magnetostrictive ferromagnetic material enclosed withina container 62 composed of a ferrimagnetic filled plastic, such asbarium ferrite in polyester consisting of two parts: a boat 60 and acover 44. The container must be constructed in such a manner that thestrip 18 remains undamped or free to vibrate upon being placed in theboat 60 and enclosed by the cover 44. This can be accomplished byleaving approximately 1 millimeter clearance on all inside dimensions.Also the container 62 must enable sound waves produced by themechanically vibrating strip 18 to efficiently escape to thesurroundings. One method of accomplishing this is through the use ofacoustic resonators such as those created by impregnating the container62 with a network of holes. The marker is adapted, when armed, tomechanically resonate at a frequency within the range of the incidentmagnetic field. The hard magnetic container 62 enclosing the strip 18 offerromagnetic material is adapted, upon being magnetized, tomagnetically bias the strip 18 and thereby arm it to resonate at thatfrequency. The strip 18 has a magnetomechanical coupling factor, k,greater than 0, where k= √(1-f_(r) ² /f_(a) ²) , f_(r) and f_(a) beingthe resonant and anti-resonant frequencies, respectively.

Upon exposure to the magnetic field within interrogation zone 12, marker16 is characterized by a substantial change in its acoustic output atthe resonant frequency that provides marker 16 with signal identity. Adetecting means 20 is arranged to detect changes in acoustic levelsproduced in the vicinity of the interrogation zone 12 by the presence ofmarker 16 therewithin.

Typically, the system 10 includes a coil unit 22 and an acoustictransducer 24 disposed in the proximity of a path leading to the exit 26of a store. Detection circuitry, including an alarm 28, is housed withina cabinet 30 located near the exit 26. Articles of merchandise 19 suchas wearing apparel, appliances, books and the like are displayed withinthe store. Each of the articles 19 has secured thereto a marker 16constructed in accordance with the present invention. As shown in FIG.4, the marker 16 includes a magneto-strictive ferromagnetic strip 18that is normally in an activated mode. When marker 16 is in theactivated mode, placement of an article 19 in the proximity of the coilunit 22 and acoustic transducer 24 of interrogation zone 12 will causean alarm to be emitted from cabinet 30. In this manner, the system 10prevents unauthorized removal of articles of merchandise 19 from thestore.

Disposed on a checkout counter near cash register 36 is a deactivatorsystem 38. The latter can be electrically connected to cash register 36by wire 40. Articles 19 that have been properly paid for are placedwithin an aperture 42 of deactivation system 38, whereupon a magneticfield is applied to marker 16. The deactivation system 38 has detectioncircuitry adapted to activate a desensitizing circuit in response toacoustic signals generated by marker 16. The desensitizing circuitapplies to marker 16 a magnetic field that places the marker 16 in adeactivated mode, by either increasing or decreasing the magnetic biasfield strength of the hard magnetic container 62, by an amountsufficient to move the resonant frequency (fr) outside of the frequencyrange of the applied field or to decrease the acoustic output levelsufficiently to make it undetectable. The article 19 carrying thedeactivated marker 16 may then be carried through interrogation zone 12without triggering the alarm 28 in cabinet 30.

The theft detection system circuitry with which the marker 16 isassociated can be any system capable of (1) generating within aninterrogation zone an incident magnetic field of desired frequency, (2)detecting changes in acoustic signals at frequencies produced in thevicinity of the interrogation zone by the presence of the marker and (3)distinguishing the particular resonant changes in acoustic output of themarker from other variations in signals detected.

Such systems typically include means for transmitting a varyingelectrical current from an oscillator and amplifier through conductivecoils that form a frame antenna capable of developing a varying magneticfield.

In accordance with a preferred embodiment of the invention, marker 16 iscomposed of a magnetostrictive amorphous metal alloy. The marker is inthe form of a strip having a first component composed of a compositionconsisting essentially of the formula M_(a) N_(b) O_(c) X_(d) Y_(e)Z_(f), where M is at lease one of iron and cobalt, N is nickel, O is atleast one of chromium and molybdenum, X is at least one of boron andphosphorous, Y is silicon, Z is carbon, "a"-"f" are in atom percent, "a"ranges from about 35-85, "b" ranges from about 0-45, "c" ranges fromabout 0-7, "d" ranges from about 5-22, "e" ranges from about 0-15 and"f" ranges from about 0-2, and the sum of d+e+f ranges from about 15-25.

It has been found that a strip 18 of material having the formulaspecified above is particularly adapted to resonate mechanically at apreselected frequency of an incident magnetic field. While we do notwish to be bound by any theory, it is believed that, in markers of theaforesaid composition, direct magnetic coupling between ac magneticfield and the marker 16 occurs by means of the following mechanism.

When a ferromagnetic material such as an amorphous metal ribbon is in amagnetic field (H), the ribbon's magnetic domains are caused to growand/or rotate. This domain movement allows magnetic energy to be stored,in addition to a small amount of energy which is lost as heat. When thefield is removed, the domains return to their original orientationreleasing the stored magnetic energy, again minus a small amount ofenergy lost as heat. Amorphous metals have high efficiency in this modeof energy storage. Since amorphous metals have no grain boundaries andhave high resistivities, their energy losses are extraordinarily low.

When the ferromagnetic ribbon is magnetostrictive, an additional mode ofenergy storage is also possible. In the presence of a magnetic field, amagnetostrictive amorphous metal ribbon will have energy storedmagnetically as described above but will also have energy storedmechanically via magnetostriction. The additional mode of energy storagemay be viewed as an increase in the effective magnetic permeability ofthe ribbon.

When an ac magnetic field and a dc field are introduced on themagnetostrictive ribbon (such as can be generated by and ac and dcelectric currents in a solenoid), energy is alternatively stored andreleased with the frequency of the ac field. The magnetostrictive energystorage and release are maximal at the material's mechanical resonancefrequency and minimal at its anti-resonance. This energy storage andrelease induces a voltage in a pickup coil via flux density changes inthe ribbon. The flux density change may also be viewed as an increase ineffective magnetic permeability at the resonance frequency and adecrease at anti-resonance, thus, in effect, increasing or decreasing,respectively, the magnetic coupling between the driving solenoid and asecond pickup solenoid. The voltage induced by the purely magneticenergy exchange is linear with frequency and the change in voltage withfrequency is small over a limited frequency range. The voltage inducedby the mechanical energy exchange is also linear with frequency exceptnear mechanical resonance. For a thin ribbon the mechanical resonancefrequency is given by: ##EQU1## where L, E and D are the length, Youngsmodulus and mass density of the ribbon. Therefore, when the frequency ofthe ac magnetic field is swept around f_(r), a characteristic signatureis generated. The resonance peak is closely followed by ananti-resonance peak shown in FIG. 3. This anti-resonant peak occurs whenthe mechanical energy storage is near zero.

The transfer of magnetic and mechanical energy described above is calledmagnetomechanical coupling (MMC), and can be seen in allmagnetostrictive materials. The efficiency of this energy transfer isproportional to the square of the magnetomechanical coupling factor (k),and is defined at the ratio of mechanical to magnetic energy.Phenomenologically, k is defined as k=√(1-f_(r) ² /f_(a) ²) where f_(r)and f_(a) are the resonant and anti-resonant frequencies describedabove. The larger the k factor, the greater the voltage differencebetween resonant peak and anti-resonant valley. The mechanicalvibrations produced by a magnetostrictive ribbon at resonance aregreatly enhanced by large k values. Thus acoustic detection utilizingsound waves resulting from such mechanical vibrations is facilitated bylarge k values. Also, the larger the k, the larger the difference infrequency between resonance and anti-resonance. Therefore, a large kfacilitates the observation of the MMC phenomena.

Coupling factors are influenced in a given amorphous metal by the levelof bias field present, the level of internal stress (or structuralanisotropy) present and by the level and direction of any magneticanisotropy. Annealing an amorphous metal relieves internal stresses,thus enhancing k. The structural anisotropy is small due to the ribbon'samorphous nature, also enhancing k. Annealing in a properly orientedmagnetic field can significantly enhance coupling factors. Domainmovement can be maximized when the ribbon has a magnetic anisotropywhich is perpendicular to the interrogating field. Because ofdemagnetizing field effects, it is practical to interrogate the ribbononly along its length (this being the longest dimension). Therefore, theinduced magnetic anisotropy should be transverse to the long dimensionof the ribbon in a saturating magnetic field which is perpendicular toribbon length (cross-field annealed). For a 1/2 inch ribbon, a field ofa few hundred oersted is required. The optimum time and temperature ofthe anneal depends on the alloy employed. As an example, aniron-boron-silicon alloy yields an optimum coupling (k>0.90) whencross-field annealed at 400° C. for 30 minutes. This anneal yields anoptimum bias field of 1 Oe. For alloys having the compositions specifiedhereinabove, the annealing temperature ranges from about 300° to 450° C.and the annealed time ranges from about 7 to 120 min.

The anneal also effects the bias field required to optimize k. For agiven amorphous metal with a given anneal, the coupling depends stronglyon the bias field. At zero and saturating fields, the coupling is zero(no resonant and anti-resonant phenomena). For a given alloy, an optimumbias field exists which yields a maximum k. For alloys having thecompositions specified herein, the bias field required to optimize kranges from about 0.1 to 20 Oe.

Even though most magnetostrictive materials will exhibit some MMC,amorphous metals yield extremely high coupling factors, and are,therefore highly preferred. As-cast amorphous metals yield higher k thanmost other magnetostrictive materials. No material has higher k thanamorphous metals when cross-field annealed. Amorphous metals have high kbecause they have:

(a) low magnetic losses (no grain boundries, high resistivity), (b) lowstructural and stress anisotropy, (c) reasonable magnetostriction and(d) can be given a beneficial magnetic anisotropy.

Amorphous metal alloys make good targets because (a) they have highk--even as-cast, (b) they are mechanically strong, tough and ductile,(c) they require low bias fields and (d) they have extremely highmagnetostrictivity (they develop a large force upon resonating and are,therefore, more difficult to damp out). It will be appreciated,therefore, that the amorphous metals of which the marker of thisinvention is composed need not be annealed, but may be incorporated intothe marker "as cast".

Examples of amorphous ferromagnetic marker compositions in atomicpercent within the scope of the invention are set forth below in Table1.

                  TABLE 1                                                         ______________________________________                                        ALLOY     AS-CAST k   OPTIMAL ANNEALED k                                      ______________________________________                                        Fe.sub.78 Si.sub.9 B.sub.13                                                             0.35        >0.90                                                   Fe.sub.79 Si.sub.5 B.sub.16                                                             0.31        >0.90                                                   Fe.sub.81 B.sub.13.5 Si.sub.3.5 C.sub.2                                                 0.22        >0.90                                                   Fe.sub.67 Co.sub.18 B.sub.14 Si.sub.1                                                   0.45         0.72                                                   Fe.sub.40 Ni.sub.38 Mo.sub.4 B.sub.18                                                   0.23         0.59                                                   ______________________________________                                    

Examples of amorphous metals that have been found unsuitable for use asarticle surveillance system markers as set forth in Table 2.

                  TABLE 2                                                         ______________________________________                                        COMPOSITION PERCENT                                                           EXAMPLE 1               EXAMPLE 2                                             ______________________________________                                        Ni at. %    71.67       Ni at. % 65.63                                        wt. %       84.49       wt. %    76.97                                        Cr at. %    5.75        Cr at. % 11.5                                         wt. %       6           wt. %    12.0                                         B at. %     12.68       B at. %  11.58                                        wt. %       2.75        wt. %    2.5                                          Si at. %    7.10        Si at. % 7.13                                         wt. %       4           wt. %    4                                            Fe at. %    2.23        Fe at. % 3.14                                         wt. %       2.5         wt. %    3.5                                          C at. %     .25         C at. %  .12                                          wt. %       .06         wt. %    .03                                          P at. %     .032        P at. %  --                                           wt. %       .02         wt. %    --                                           S at. %     .031        S at. %  --                                           wt. %       .02         wt. %    --                                           Al at. %    .093        Al at. % --                                           wt. %       .05         wt. %    --                                           Ti at. %    .052        Ti at. % --                                           wt. %       .05         wt. %    --                                           Zr at. %    .027        Zr at. % --                                           wt. %       .05         wt. %    --                                           Co at. %    .085        Co at. % .85                                          wt. %       .1          wt. %    1.0                                          ______________________________________                                    

The amorphous ferromagnetic metal marker of the invention is prepared bycooling a melt of the desired composition at a rate of at least about10⁵ ° C./sec, employing metal alloy quenching techniques well-known tothe amorphous metal alloy art; see, e.g., U.S. Pat. No. 3,856,513 toChen et al. The purity of all compositions is that found in normalcommercial practice.

The variety of techniques are available for fabricating continuousribbon, wire, sheet, etc. Typically, a particular composition isselected, powders or granules of the requisite elements in the desiredportions are melted and homogenized, and the molten alloy is rapidlyquenched on a chill surface, such as a rapidly rotating metal cylinder.

Under these quenching conditions, a metastable, homogeneous, ductilematerial is obtained. The metastable material may be amorphous, in whichcase there is no long-range order. X-ray diffraction patterns ofamorphous metal alloy show only a diffuse halo, similar to that observedfor inorganic oxide glasses. Such amorphous alloys must be at least 50%amorphous to be sufficiently ductile to permit subsequent handling, suchas stamping complex marker shapes from ribbons of the alloys withoutdegradation of the marker's signal identity. Preferably, the amorphousmetal marker must be at least 80% amorphous to attain superiorductility.

The metastable phase may also be a solid solution of the constituentelements. In the case of the marker of the invention, such metastable,solid solution phases are not ordinarily produced under conventionalprocessing techniques employed in the art of fabricating crystallinealloys. X-ray diffraction patterns of the solid solution alloys show thesharp diffraction peaks characteristics of crystalline alloys, with somebroadening of the peaks due to desired find-grained size ofcrystallites. Such metastable materials are also ductile when producedunder the conditions described above.

The magnetostrictive strip 18 of which marker 16 is comprised isadvantageously produced in foil (or ribbon) form, and may be used intheft detection applications as cast, whether the material is amorphousor a solid solution. Alternatively, foils of amorphous metal alloys maybe heat treated to obtain a crystalline phase, preferably find-grained,in order to promote longer die life when stamping of complex markershapes is contemplated.

The amorphous ferromagnetic material of strip 18 is exceedingly ductile.By ductile is meant that the strip 18 can be bent around a radius assmall as ten times the foil thickness without fracture. Such bending ofthe strip 18 produces little or no degradation in magnetic propertiesgenerated by the marker upon application of the interrogating magneticfield thereto. As a result, the marker retains its signal identitydespite being flexed or bent during (1) manufacture (e.g., cutting,stamping or otherwise forming the strip 18 into the desired length andconfiguration) and, optionally, applying hard magnetic biasing magneticsthereto to produce an on/off marker, (2) application of the marker 16 tothe protected articles 19, (3) handling of the articles 19 by employeesand customers and (4) attempts at signal destruction designed tocircumvent the system 10.

There are numerous alternate marker configurations in which an acousticsignal is produced by interrogating such markers at their resonantfrequency. These alternate marker configurations can be summarized bydescribing two general catagories. One catagory entails a marker 16wherein a strip 18 is disposed adjacent to a ferromagnetic element 44,such as a biasing magnetic capable or applying a dc field to strip 18.The biasing magnetic has a configuration and disposition adapted toprovide strip 18 with a single pair of magnetic poles, each of the polesbeing at opposite extremes of the long dimension of strip 18. Thecomposite assembly is then placed within the hollow recess 60 of a rigidcontainer 62 impregnated with a network of holes composed of polymericmaterial such as polyethylene or the like, to protect the assemblyagainst mechanical damping and to allow sound waves to efficientlyescape to the surroundings. The biasing magnet 44 is typically a flatstrip of high coercivity material such as SAE 1095 steel, Vicalloy,Remalloy or Arnokrome. Biasing magnet 44 is a hard magnetic elementwhich can, alternatively, comprise a crystalline region of the amorphousmaterial. Such biasing magnet 44 is held in the assembly in a parallel,adjacent plane, such that the high coercivity material does not causemechanical interference with the vibration of the strip 18. Generally,biasing magnet 44 acts as one surface of the package. Alternatively, twopieces of high magnetic coercivity material may be placed at either endof strip 18, with their magnetic poles so arranged as to induce a signalpole-pair therein. This configuration of the assembly is thinner butlonger than that utilizing a signal piece of high coercivity material inan adjacent parallel plane to the permeable strip. Alternatively thebias field can be supplied by an external field coil pair disposedremotely from the marker in the exit or aisleway. In this embodiment,the biasing magnet made of high coercivity material would not berequired. Such a marker is not readily deactivated in the manner ofmarkers equipped with biasing magnet 44. Further biasing magnet 44 cancomprise a plurality of pieces of high magnetic coercivity material, asin the order of up to 10 or more pieces, disposed longitudinally ofstrip 18.

The second category (FIG. 5) entails a marker 47 wherein the strip 18 isdisposed to become the driver of an acoustic reasonator. Theseconfigurations include any marker in which the strip 18 is physicallyadhered to a block 51 of structurally rigid material. Suchconfigurations produce acoustic signals at frequencies other than theresonant frequency of the strip 18 resulting from the mechanicalvibrations of the attached block 51. The means of biasing such markersremains the same as that used in the previously described markerconfiguration. The advantage of this configuration lies in thesimplicity of construction and the ability to greatly alter thedetectable frequency range of the marker. The main drawback in usingsuch configurations stems from the decreased signal to noise ratioresulting from the increased mass of the resonator.

Unlike markers which generate harmonics of the interrogation frequencyin a pickup coil, resonant frequency markers generate a distinctiveincrease in the acoustic level induced in the acoustical detectionsystem when the primary or drive frequency equals the resonantfrequency. In the case of harmonic generating markers, the feature whichdistinguishes the presence of the high magnetic permeability material inthe marker from other ferromagnetic materials in the generation ofharmonics of high order. Hence, in order to distinguish between the twomaterials, detection of the presence of these high order harmonics isrequired. Typically, the voltage of high order harmonics is only a fewpercent of the voltage of the primary or drive frequency.

In contrast, the resonant frequency marker of the present invention isdistinguished from other objects by the particular signal shapegenerated by the marker when the drive frequency passes through theresonant frequency of the marker. Requiring that a bias field be presentalso facilities the process of distinguishing the marker from otheritems. The marked effect upon the fundamental acoustic level induced inthe acoustical detection system by the marker makes it easy to detect inthe presence of other objects. Note that such markers can be detected byelectromagnetic means such as those described in U.S. patentapplication, Ser. No. 373,061, filed Apr. 29, 1982 as well as acousticmeans, however, markers described in the aforementioned patentapplication produce acoustic signals too weak to be detected atdistances greater than about 1/2 meter. The acoustic markers describedherein can operate with the acoustic transducer displaced by a distancein excess of six meters (20 feet) from these markers. Acoustic markersenable the detection system to continuously monitor the markers outputas a result of the differing modes of interrogation and detection ofsuch markers. Markers in which the detection system and interrogationsystem utilize the same mode of interaction creates differentiatingdetection problems between the interrogation systems output and themarkers output. FIG. 3 shows the increase in induced voltage in a pickupcoil caused by a marker when the interrogating field is swept around theresonant frequency of the marker. The acoustic marker's output issimilar in characteristics to those of the aforesaid U.S. patentapplication, Ser. No. 373,061, filed Apr. 29, 1982 as shown in FIG. 3with the exception of the anti-resonant frequency, which is obscured byacoustic background noise levels. This voltage increase occurs only whenthe marker is subjected to a magnetic field whose frequency is equal tothe resonant frequency of the marker.

In operation, the system is equipped with an interrogation and detectioncircuit, shown in FIG. 6. A variable frequency or single frequencyoscillator 100 the output frequency of which matches that of the markersto be employed drives an amplifier 110 whose output is applied to aninterrogation coil 120 such that an ac field is developed in the spacethrough which the marker 16 and other materials are to pass. Theinterrogation coil 120 is so configured as to provide an essentiallyuniform flux density in the interrogation zone. This may be accomplishedby means of a Helmholtz configuration, or some other suitablearrangement. The amplifier 110 has its impedence matched with that ofinterrogating coil 120 to maximize the efficiency of amplifier 110 andthereby minimize the power requirements thereof.

The acoustic detection system 300 comprises a detector-transducer(microphone) 140 whose output is conditioned by a narrow band passfilter 150 and then amplified by a power amplifier 155. The amplifiedconditioned signal is then applied to the input of the detector 170. Asignal level above the threshhold is created by the presence of a marker16 within the interrogation zone 12 causing an alarm signal to begenerated by the detector 70. Demagnetization of the biasing magnet bythe clerk, upon checkout, alters the resonant frequency and preventsdetection.

The magnetude of the filtered, amplified signal for a variety ofarticles placed in the interrogation zone depicted in FIG. 6 is setforth in Table 3 below:

                  TABLE 3                                                         ______________________________________                                        Material (atom %)                                                                            Structure   Signal                                             ______________________________________                                        Fe(51), Ni(49) microcrystalline                                                                          less than 50 mV                                    Ni             "           "                                                  (NiZn).sub.0.5 Fe.sub.2 O.sub.4 *                                                            "           "                                                  Fe.sub.40 Ni.sub.38 Mo.sub.4 B.sub.18                                                        amorphous   120 V                                              ______________________________________                                         *chemical formula                                                        

The article surveillance system 10 which has been disclosed herein can,of course, be modified in numerous ways without departing from the scopeof the invention. For example, the hard magnetic material that suppliesdc bias to activate the marker may alternatively be used to magneticallysaturate the magnetostrictive strip 18, and thereby deactivate themarker 16. The dc bias may be generated (1) by an electric coil, (2) bythe earth's field or (3) by utilizing the remanent flux in themagnetostrictive material of strip 18. Instead of interrogating themarker 16 with a constant frequency, interrogation can entailcontinuously sweeping the interrogation frequency about the resonantfrequency of the marker to induce the marker to vibrate, and thereafterdetecting the substantial change in acoustic levels occurring at themechanical resonance frequency of the marker. Instead of continuouslysweeping tne interrogation frequency about the resonant frequency of themarker to induce the marker to vibrate, and thereafter detecting thesubstantial change in acoustic levels occurring at the mechanicalresonance frequency of the marker. Additionally an interrogating pulseor burst may be used to excite the marker into oscillation. After aninterrogating signal of the pulse or burst type is over, the marker willundergo damped oscillation at its resonance frequency. The vibratingmarker will cause an acoustic signal to be detected in the acoustictransducer (microphone) at the resonant frequency. Several types ofsignals can be used to energize the marker. For example, the marker maybe energized by a single having the form of a signal frequency sine waveburst, the frequency of which is centered at the markers naturalresonance. Other similar modifications can be made which fall within thescope of the present invention. It is accordingly intended that allmatter contained in the above description and shown in the accompanyingdrawings be interpreted as illustrative and not in a limiting sense.

More specifically, there is illustrated in FIG. 7 an alternative systemfor interrogating and detecting the marker 16. Synchronizing circuit 200controls the operation of energizing circuit 201 and receiving circuit202. The synchronizing circuit 200 sends a synchronizing gate pulse tothe energizing circuit 201 which activates the energizing circuit 201.Upon being activated the energizing circuit 201 generates and sends aninterrogation signal to interrogating coil 206 for the duration of thesynchronizing pulse. An interrogating magnetic field generated by thecoil 206 excites marker 16 into mecnanical resonance. Upon completion ofthe interrogating signal, the synchronizing circuit 200 produces a gatepulse to the receiver circuit 202, which activates the receiver circuit202. During the period that receiver circuit 202 is activated, themarker if present, will generate a signal at the frequency of mechanicalresonance of the marker in the acoustic transducer (microphone) 207.When the marker frequency is sensed, by receiver 202, the receiverapplies a voltage to indicator 203, which records the presence of themarker 16.

The interrogating signal generated by energizing circuit 201 may be asingle frequency sine wave burst whose frequency is centered at themarkers natural resonance. Alternatively, the interrogating signal maybe an impulse whose width is less than or equal to 1/(2f_(r)), wheref_(r) is the marker's resonant frequency. In yet another embodiment ofthe invention, the interrogating signal may be a burst of noise or acomposite signal whose frequency spectrum contains the resonantfrequency of the marker.

Having thus described the invention in rather full detail, it will beunderstood that such detail need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoined claims.

What we claimed is
 1. For use in a magnetic article surveillance system,a marker which, when armed, is adapted to resonate at a frequencyprovided by an incident magnetic field applied within an interrogationzone and to have a substantial change in acoustic output level at saidfrequency that provides said marker with signal identity, said markerbeing a strip of magnetostrictive ferromagnetic material disposedadjacent to a hard magnetic element which, upon being magnetized,magnetically biases said strip and arms it to resonate at saidfrequency, said strip having magnetomechanical coupling factor, kgreater than 0, where k=√(1-f_(r) ² /f_(a) ²), f_(r) and f_(a) being theresonate and anti-resonant frequencies, respectively.
 2. A marker asrecited in claim 1, wherein said material is at least 50 percentamorphous.
 3. A marker as recited in claim 1, wherein said material isat least 80 percent amorphous.
 4. A marker as recited in claim 1,wherein said hard magnetic element has a coercivity higher than that ofsaid magnetostrictive material.
 5. A marker as recited in claim 1,wherein said hard magnetic element is adapted to be magnetized to armsaid strip and thereby increase the resonance thereof at said frequency.6. A marker as recited in claim 5, wherein said hard magnetic elementcomprises a crystalline region of said amorphous material.
 7. A markeras recited in claim 5, wherein said increase in resonance of said markerprovides it with signal identity.
 8. A marker as recited in claim 1,wherein said hard magnetic element is in the form of a container inwhich said strip is enclosed in such a manner as to allow said strip tovibrate freely.
 9. A marker as recited in claim 8, wherein saidcontainer is constructed in such a manner as to efficiently transmitacoustic signals.
 10. A marker as recited in claim 1, wherein said stripis enclosed within a nonmagnetic container in such a manner as to allowto strip to vibrate freely while efficiently transmitting acousticsignals.
 11. A marker as recited in claim 10, wherein said hard magneticelement is affixed to said container.
 12. A marker as recited in claim2, wherein said material has a composition consisting essentially of theformula M_(a) N_(b) O_(c) X_(d) Y_(e) Z_(f), where M is at least one ofiron and cobalt, N is nickle, O is at least one of chromium andmolybdenum, X is at least one of the boron and phosphorous, Y issilicon, Z is carbon, "a"-"f" are in atom percent, "a" ranges from about35-85, "b" ranges from about 0-45, "c" ranges from about 0-7, "d" rangesfrom about 5-22, "e" ranges from about 0-15 and "f" ranges from about0-2, and the sum of d+e+f ranges from about 15-25.
 13. A marker asrecited in claim 4, wherein said hard magnetic element comprises aplurality of pieces of high magnetic coercivity material.
 14. For use ina magnetic article surveillance system, a marker adapted to resonate ata frequency provided by an incident magnetic field applied within aninterrogation zone, and having a substantial change in acoustic outputat said frequency that provides said marker with signal identity, saidmarker being a strip of magnetostrictive ferromagnetic material adaptedto be magnetically biased and thereby armed to resonate at saidfrequency, said strip having a magnetomechanical coupling factor, k,greater than 0, where k=√(1-f_(r) ² /f_(a) ²), f_(r) and f_(a) being theresonant and anti-resonant frequencies, respectively.
 15. A marker asrecited in claim 14, wherein said material is at least 50 percentamorphous.
 16. A marker as recited in claim 14, wherein said material isat least 80 percent amorphous.
 17. A marker as recited in claim 14,wherein said marker has at least one hard magnetic element disposedadjacent thereto and adapted to bias said strip and arm it to resonateat said frequency.
 18. A marker as recited in claim 14, wherein saidhard magnetic element has coercivity higher than said magnetostrictivematerial.
 19. A marker as recited in claim 17, wherein said hardmagnetic element has coercivity higher than said amorphous material. 20.A marker as recited in claim 19, wherein said hard magnetic element isadapted to be magnetized to disarm said strip and thereby decreaseresonance thereof at said frequency.
 21. A marker as recited in claim20, wherein said decrease in resonance of said marker causes it to loseits signal identity.
 22. A marker as recited in claim 15, wherein saidmaterial has a composition consisting essentially of the formula M_(a)N_(b) O_(c) X_(d) Y_(e) Z_(f), where M is at least one of iron andcobalt, N is nickel, O is at least one of chromium and molybdenum, X isat least one of boron and phosphorous, Y is silicon, Z is carbon,"a"-"f" are in atom percent, "a" ranges from about 35-85, "b" rangesfrom about 0-45, "c" ranges from about 0-7, "d" ranges from about 5-22,"e" ranges from about 0-15 and "f" ranges from about 0-2, and the sum ofd+e+f ranges from about 15-25.
 23. A marker as recited in claim 14,wherein said strip is adapted to mechanically vibrate a block ofstructurally rigid material upon being affixed to said block.
 24. Amarker as recited in claim 23, wherein said hard magnetic element isaffixed to said block.
 25. A marker as recited in claim 14, wherein saidincident magnetic field is of continuous frequency corresponding to saidmarker's resonant frequency.
 26. A marker as recited in claim 14,wherein said incident magnetic field is swept to provide said frequency.27. A marker as recited in claim 14, wherein said frequency is in theform of a pulse.
 28. A marker as recited in claim 27, wherein said pulsehas a width less than or equal to 1/(2f_(r)), where f_(r) is theresonant frequency of said strip.
 29. An article surveillance systemresponsive to the presence of a marker within an interrogation zone,comprising:a. means for defining an interrogation zone; b. generatingmeans for generating a magnetic field having a frequency band withinsaid interrogation zone, said generating means including aninterrogating coil, c. a marker secured to an article appointed forpassage through said interrogation zone, said marker being characterizedby a substantial change in its acoustic output level within saidfrequency band that provides said marker with signal identity, andcomprising an elongated ductile strip of magnetostrictive, ferromagneticmaterial adapted to be magnetically biased and thereby armed to resonatemechanically at a frequency within the frequency band of said magneticfield, said strip having a magnetomechanical coupling factor, k, greaterthan 0, wherein k=√(1-f_(r) ² /f_(a) ²), f_(r) and f_(a) being theresonant and anti-resonant frequencies, respectively; and e. detectingmeans for detecting resonance of said marker, within said interrogationzone at said resonant frequency and distinguishing said change in itsacoustic output level from changes in acoustic output at other than theresonant frequency of said marker.
 30. An article surveillance system asrecited in claim 29, wherein said generating means includes energizingmeans adapted to provide a continuous signal at the resonant frequencyof said marker.
 31. An article surveillance system as recited in claim29 wherein said generating means includes frequency sweeping meansadapted to sweep through the resonant frequency of said marker.
 32. Anarticle surveillance system as recited in claim 29, wherein saidgenerating means includes energizing means adapted to provide saidinterrogating coil with a burst of single frequency sine wave.
 33. Anarticle surveillance system as recited in claim 29, wherein saidgenerating means includes energizing means adapted to provide saidinterrogating coil with a pulse, the width of which is less than orequal to 1/(2f_(r)), where f_(r) is the marker's resonant frequency. 34.An article surveillance system as recited in claim 29, wherein saidgenerating means includes energizing means adapted to provide saidinterrogation coil with a burst of noise.
 35. An article surveillancesystem as recited in claim 29, wherein said generating means includesenergizing means adapted to provide said interrogating coil with a sweptfrequency sine wave.
 36. An article surveillance system as recited inclaim 29, wherein said detecting means includes receiving means fordistinguishing the frequency of marker resonance detected within saidinterrogation zone from other frequencies induced therein.
 37. Anarticle surveillance system as recited in claim 36, wherein saiddetection system includes an acoustic transducer adapted to monitor saidmarker's resonant frequency.